26.2 Stars

If you move away from a street light or a star, it
shines just as brightly as before—but to you it
appears fainter. Absolute brightness is how bright a
star really is. A star’s absolute brightness is a characteristic
of the star and does not depend on how far it
is from Earth. You can calculate a star’s absolute
brightness if you know its distance from Earth and
its apparent brightness.
Size and Mass Once astronomers know a star’s
temperature and absolute brightness, they can estimate
its diameter and then calculate its volume.
However, there is no direct way of finding the mass of
an isolated star. Instead, astronomers are able to calculate
the masses of many stars by observing the
gravitational interaction of stars that occur in pairs.
From such observations, astronomers have determined
that, for most stars, there is a relationship
between mass and absolute brightness. Astronomers
have found that many stars are similar to the sun in
size and mass.
How can astronomers determine a star’s mass?
Composition A spectrograph is an instrument that spreads light
from a hot glowing object, such as a light bulb or a star, into a spectrum.
Astronomers can use spectrographs to identify the various
elements in a star’s atmosphere.
Each star has its own spectrum. The elements within a star’s atmosphere
absorb light from the star’s photosphere. Each element absorbs
light of different wavelengths, removing these wavelengths from the
star’s continuous spectrum. The result is a bright spectrum, such as
the one shown in Figure 12. It contains a set of dark lines called
absorption lines that show where light has been absorbed. Just as fingerprints
can be used to identify a person, a star’s absorption lines can
be used to identify different elements in the star.
Absorption lines of most elements have been identified in
the spectra of stars. Observations of such lines in many stars
have shown that the composition of most stars is fairly similar.
Most stars have a chemical makeup that is similar to the sun,
with hydrogen and helium together making up 96 to 99.9 percent
of the star’s mass.
Facts and Figures
Discovery of Helium In 1868, a spectrometer
was used for the first time to study the sun
during a solar eclipse. The French astronomer
Pierre Janssen analyzed the resulting spectrum
and noticed that a bright yellow spectral line did
not match any known elements. He proposed
that the line was associated with a new element.
Figure 11 These streetlights all
have about the same absolute
brightness. Inferring Why do
the nearby streetlights appear
brighter than the distant ones?
Figure 12 This is the spectrum of
a star. The dark absorption lines
indicate the presence of various
elements in the star.
Exploring the Universe 837
It was named helium, which stems from the
Greek word helios, meaning “sun.” Helium, a
light noble gas, is extremely difficult to detect
on Earth. In 1895, Sir William Ramsay was finally
able to demonstrate the presence of helium in
ores by treating them with acids.
Build Science Skills
Communicating Results The Stefan-
Boltzmann Law relates a star’s absolute
brightness to its size and temperature.
Written in equation form, the law states
that L 4πR 2 sT 4 where L is luminosity
(or absolute brightness), R is the radius
of a star, s is a constant, and T is temperature.
The equation can be used to
determine a star’s size if its temperature
and absolute brightness are known.
Have students research and write brief
reports about how scientists use the
Stefan-Boltzmann Law.
Verbal
Integrate Chemistry
L2
L2
Tell students that at the surface of stars
with relatively low temperatures, atoms
retain all of their electrons and radiate
neutral, or normal, spectra. At very high
temperatures, atoms collide and lose
electrons and become thermally ionized.
Ions have different spectra than atoms.
Ask, How is it possible that two stars
can have a similar composition, but
radiate different spectra? (One of the
stars is much hotter than the other.)
Logical
Answer to . . .
Figure 10 Sirius A because it is blue
Figure 11 The nearby streetlights
have a greater apparent brightness.
By observing the gravitational
interaction of
stars that appear in pairs, which led
to the determination that mass and
absolute brightness are related
Exploring the Universe 837